KiCAD is a free/open-source PCB design suite. While I have not used it myself yet, I believe it is similar in capability to Eagle PCB, and there are scripts to convert Eagle libraries and designs to KiCAD format. There is a site for component libraries in KiCad format here.
kicadocaml is an alternative layout tool for Kicad.
xml4pcb coverts PCB123 design to Kicad board file.
csv2footprint helps make KiCad PCB footprints from spreadsheets.
Cern-KiCad is an effort to improve KiCad by Cern. I don't know if this goes up to mainline KiCad or not, but it sounds like they are working on some impressive improvements to KiCad.
Pulsonix is, I'm told, a higher-end PCB/schematic tool with FPGA and high-speed design features, and I'm told it costs about twice what Eagle does. I've not asked for a quote myself yet, so I'm just repeating what I've heard in SparkFun forums. Sounds like a decent tool.
UpVerter is a free online schematic, PDB and simulation tool for open-source projects. Share your projects, fork others, share and fork symbol and footprint libraries, etc.
Autodesk 123D Circuits (previously known as Circuits.IO) is another online PCB tool
CircuitHub is a place for people to freely share footprints and symbols. If you need something, check to see if someone has already provided it from their project.
Ultra Librarian from Accelrated Designs is a tool-agnostic format for symbol/footprint libraries. It's commercial, but not horribly expensive (Lite version), exports to a variety of PCB tools such as Eagle, and I've seen a handful of component vendors offering their part libraries in this format now.
Autorouters
FreeRouting is a free to use PCB interactive and autorouter compatible with Eagle and KiCad. It is a web-based Java application that interacts with the PCB software. It is not open-sourced, and it is not downloadable/installable to your computer, it only works over the web.
qautorouter intends to be a Specctra format compatible PCN autorouter.
Electra autorouter tool that can be used with Eagle and other layout tools. Has a relatively lower price than some high-end tools, but ain't free.
Sierra Circuits provides free online tools for HDI Layer Stackup and Material Selection tools to aid in design. Not sure, but this may be somewhat specific to their own fabrication service.
OpenEMS is an open-source 3d Field Solver that can import HyperLynx .hyp PCB files. It interfaces with Matlab/Octave, and uses hyp2mat to import Hyperlynx format files.
Note that most gerber viewers only do the rs274X format, not the rs274D stuff. The gerbv mailing list had a discussion about this and a perl script came out of it, and was later updatedas someone had issues. I used this successfully looking at some D gerbers from PLX.
Gerbv is a free/open-source Gerber file viewer tool and is part of the Geda (aka GPLeda) tool set.
Viewmate is a gerber viewer with a free cost version available.
GraphiCode offers a free cost version of their GCPrevue gerber viewer. The pay-for Plus version allows some editing and other additional features.
GBtiler is a tool to tile gerber designs together, for RS274X.
Similarly, there is a free Altium viewer. I haven't tried it to see if there is any ASCII dump or anything, or if Altium import has already been solved. I don't think I have any Altium designs layout or schematic to view anyway, but good to know about if I do. They also list a free P-CAD viewer, and a PADS to P-Cad converter, to effect a free PADS viewer, should that be needed.
Zuken provides a free evaluation version of their CADStar product called CADStar Express. It is limited to 300 pinsand/or 50 components, and lacks some of the high-end design features such as signal integrity based routing.
Mentor provides a free viewer for the odb++ format. Looks liek Mentor bought Valor, and thus Mentor owns the proprietary obd++ spec. You can obtain a copy of the spec under NDA and make a converter for Eagle or Kicad. :) Maybe create a library in Java, to be used by any tool on any platform, while staying closed-source for NDA reasons. I wish I had time... Artwork has obd++ to something else converters, but with some people being asked for obd++ databases by their fabs, it seems a gerber to obd++ converter makes more sense. Artwork also has a brief overview of what obd++ is.
Simulation
eispice is open-source spice simulator that can directy import Ibis models
PCB EDA Tools
PCB tools
PCB Information/Books/Webinars
Fabricators
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